The present invention relates to the structure of a thermal printhead for use in e.g. a facsimile machine to perform printing. Further, the present invention relates to a method of forming a protective coating in such a thermal printhead.
Generally, a thermal printhead includes a head substrate as a support member. The upper surface of the head substrate is provided with a plurality of drive ICs, a predetermined wiring pattern and a linear heating resistor. The wiring pattern includes a plurality of individual electrodes respectively connected to the drive ICs and a common electrode which has a plurality of comb-tooth projections (each hereinafter referred to as a xe2x80x9ctoothxe2x80x9d). The individual electrodes extend in parallel to each other. Each tooth of the common electrode extends into a space between two adjacent individual electrodes. Thus, the individual electrodes and the teeth are alternately disposed. The heating resistor extends across the individual electrodes and the teeth. The upper surface of the head substrate is further formed with a protective coating for covering the individual electrodes, the common electrode and the heating resistor.
Conventionally, such a protective coating is made by a thick film technique using a glass material which is excellent in abrasion resistance and electric insulation. (The thick film technique is a method comprising the steps of applying a paste material onto the head substrate by screen printing, and thereafter drying and baking the applied material.) However, this kind of protective coating has the following problems. The protective glass coating generates static electricity due to friction with a recording paper. As a result, the protective coating is likely to be electrostatically charged. Thus, due to an electrostatic discharge, the heating resistor and the wiring pattern may be electrostatically damaged.
To avoid such an electrostatic discharge, a two-layer protective coating has been recently proposed. Specifically, the protective coating includes a first coating layer formed of e.g. glass on a top surface of the head substrate by a thick film technique, and a second coating layer formed on the first coating layer by a thin film technique such as spattering. The second coating layer is made of sialon having a good abrasion resistance. The sialon contains an appropriate amount of an electrically conductive material such as titanium nitride as an additive to decrease the electrically insulating ability of the second coating layer. As a result, the second coating layer becomes less likely to be electrostatically charged.
It is true that the addition of such a conductive material makes the second coating layer less likely to be electrostatically charged. However, an electrostatic charge gradually builds up in the second coating layer. As a result, an electrostatic discharge eventually occurs between the second coating layer and the heating resistor or the wiring pattern. Thus, the addition of an electrically conductive material does not completely prevent the second coating layer from being electrostatically damaged.
The two-layer protective coating has other problems. As described above, the second coating layer is formed by spattering, specifically in the following manner. First, a head substrate formed with a first coating layer is placed in a closed chamber. Then, a target made of sialon containing an appropriate proportion of titanium nitride is disposed in facing relation to the first coating layer. Finally, a target voltage is applied across the target and the head substrate.
Conventionally, the proportion of titanium nitride to sialon is 50 wt % for example. However, this results in an excessive increase of the conductivity of the second coating layer. Thus, when a voltage is applied to the heating resistor to perform printing, a considerable voltage may be applied to the second coating layer, which may cause melting of the second coating layer.
Accordingly, to prevent such melting of the second coating layer during the printing operation, the surface electrical resistance of the second coating layer needs to be increased to such a degree that may prevent charging of the second coating layer.
To increase the surface electrical resistance of the second coating layer, the proportion of titanium nitride in sialon needs to be decreased. For this purpose, spattering should be performed using a target which includes no more than 20 wt % of titanium nitride. However, when spattering is performed using such a target containing no more than 20 wt % of titanium nitride, the weight percentage of titanium nitride in the resulting second coating layer fluctuates largely. Accordingly, it is impossible to provide a second coating layer having a stable titanium nitride-sialon composition.
Conventionally, therefore, at least 25% of titanium nitride is added to sialon. In such a case, however, the surface electrical resistance of the second coating layer cannot be increased above 104 xcexa9xc2x7cm. As a result, it is impossible to reliably prevent melting of the second coating layer when a voltage is applied to the heating resistor.
It is an object of the present invention to provide a thermal printhead which is capable of solving the above-described problems.
It is another object of the present invention to provide a method of forming a protective coating which is capable of solving the above-described problems.
In accordance with a first aspect of the present invention, there is provided a thermal printhead comprising:
a head substrate;
a heating resistor provided on the head substrate;
a plurality of individual electrodes connected to the heating resistor;
a common electrode connected to the heating resistor;
a first coating layer covering the heating resistor, the individual electrodes and the common electrode;
a second coating layer formed on the first coating layer, the second coating layer comprising sialon which contains a conductive material as an additive;
characterized that the second coating layer is electrically connected to the common electrode.
With this arrangement, the second coating layer, which comprises sialon containing a conductive material as an additive, is electrically connected to the common electrode. Accordingly, even if the second coating layer is electrostatically charged due to friction with a recording paper, the static electricity escapes to the common electrode. Thus, it is possible to prevent static buildup in the second coating layer, thereby reliably preventing the heating resistor and the individual electrodes from being electrostatically damaged due to discharge of the static charge.
Preferably, the first coating layer may be formed with at least one through-hole or notch, whereby the second coating layer is electrically connected to the common electrode via the through-hole or the notch.
With this arrangement, the provision of the through-hole or the notch facilitates electrically connecting the second coating layer to the common electrode located below the first coating layer. As a result, it is possible to advantageously reduce the labor and hence the cost required for the electrical connection.
In accordance with a second aspect of the present invention, there is provided a method of forming a protective coating comprising the steps of:
forming a first coating layer on a head substrate provided with a heating resistor and a wiring pattern for covering the heating resistor and the wiring pattern; and
forming a second coating layer on the first coating layer by spattering sialon containing titanium nitride as an additive;
characterized that the spattering is performed in a nitrogen-gas-containing atmosphere.
Preferably, the spattering may be performed using a target which is made of sialon containing 25 to 40 wt % of titanium nitride, and the atmosphere may contain 10 to 40 volume % of nitrogen gas.
With the above method, the spattering is performed in a nitrogen-gas-containing atmosphere. Accordingly, the use of a target made of sialon containing a relatively large proportion of titanium nitride allows formation of a second coating layer containing a relatively low proportion of titanium nitride (and hence having a relatively large surface electrical resistance) with a stable composition.
Specifically, when the spattering is performed in an atmosphere containing 10 to 40 volume % of nitrogen gas with the use of a target made of sialon containing 25 to 40 wt % of titanium nitride, the surface resistance of the resulting second coating layer lies in the range of from 105 xcexa9xc2x7cm to 108 xcexa9xc2x7cm (see FIG. 5). It is possible to effectively prevent the heating resistor, the individual electrodes and the like from being electrostatically damaged while also preventing melting of the second coating layer.
Various features and advantages of the present invention will become clearer from the description given below with reference to the accompanying drawings.